Neuromonitoring is a subfield of clinical patient monitoring focused on measuring various aspects of brain function and on changes therein caused by neurological diseases, accidents, and drugs commonly used to induce and maintain anesthesia in an operation room or sedation in patients under critical or intensive care.
Electroencephalography (EEG) is a well-established method for assessing brain activity. When measurement electrodes are attached on the skin of the skull surface, the weak biopotential signals generated in brain cortex may be recorded and analyzed. The EEG has been in wide use for decades in basic research of the neural systems of the brain as well as in the clinical diagnosis of various central nervous system diseases and disorders.
The EEG signal represents the sum of excitatory and inhibitory potentials of large numbers of cortical pyramidal neurons, which are organized in columns. Each EEG electrode senses the average activity of several thousands of cortical pyramidal neurons.
The EEG signal is often divided into four different frequency bands: Delta (0.5-3.5 Hz), Theta (3.5-7.0 Hz), Alpha (7.0-13.0 Hz), and Beta (13.0-32.0 Hz). In an adult, Alpha waves are found during periods of wakefulness, and they may disappear entirely during sleep. Beta waves are recorded during periods of intense activation of the central nervous system. The lower frequency Theta and Delta waves reflect drowsiness and periods of deep sleep.
In clinical environment, the EEG measurement electrodes are often placed only onto the forehead of the patient, since a frontal cortex EEG is enough for most clinical applications and the forehead is a convenient measurement area from the point of view of both the patient and the nursing staff. Consequently, various electrode array systems have been developed for acquiring EEG signals from the frontal and temple areas of a patient. U.S. Pat. No. 6,394,953, for example, discloses a sensor comprising at least one measurement electrode on the forehead and at least one further electrode in the temple area of the patient.
In addition to brain waves, a surface EEG normally includes various other signal components, such as those caused by eye movements and eye blinks. The cornea of the eye is electrically positive relative to the retina and thus the eye forms an electrical dipole whose movements cause signal components to superpose onto the EEG signal. Although the eye movements and blinks that appear in an EEG signal contain information on the state of a patient, the said signal components are on the other hand considered as artifact that hampers the analysis of the brain waves. Therefore, in order to accomplish an accurate EEG analysis, eye movements/blinks need to be detected and their effect on the EEG removed.
For the detection, it is advantageous to measure the eye movements independently from the EEG signal as far as this is possible. FIG. 1 illustrates a straightforward method for detecting eye movements in all possible directions. Horizontal eye movements are detected by measuring the voltage difference between two electrodes A and B positioned on the opposite temple areas in line with the eyes. Vertical eye movements and blinks are detected by measuring the voltage difference between two further electrodes C and D positioned above and below one eye in line with the vertical axis 11 of the eye. A drawback related to this method is that the number of electrodes needed is rather high since two additional electrodes are needed for measuring the EEG (not shown in the figure).
However, some of these electrode array systems developed for acquiring EEG signals from the frontal and temple areas of a patient are designed for uncoupling the EEG signals, the electromyographic (EMG) signals, and the electro-oculographic (EOG) signals. U.S. Pat. No. 6,032,072, for example, discloses a sensor comprising one pair of closely positioned electrodes and at least one electrode widely spaced from the said pair. The idea behind this configuration is that a pair of closely positioned electrodes reflects primarily the EMG or EOG activity, while the voltage measured across a pair of well-spaced electrodes reflects primarily the EEG activity.
U.S. Pat. No. 6,950,698, in turn, discloses an electrode array comprising at least four measurement electrodes positioned on the forehead and possibly also in the temple area of the patient. In one embodiment, an electrode pair in the top part of the forehead is primarily sensitive to EEG signal, while another pair locating above the eyebrows is primarily sensitive to EMG and EOG activity.
A further drawback related to the current electrode array systems is that the detected EMG/EOG activity may lead to a considerable loss of collected information, since the contaminated EEG epochs need to be rejected.
The present invention seeks to alleviate or eliminate the above-mentioned drawbacks and to reduce the number of electrodes needed to detect eye movements in connection with EEG measurement.